The present invention relates generally to seals between circumferentially registering endfaces of bucket tip static seal segments in steam turbine diaphragm assemblies and particularly relates to spline seals between endfaces of bucket tip static seal segments circumferentially disposed end to end in a stationary diaphragm of the steam turbine and methods of retrofitting the splined seals.
In steam turbine design, it is highly desirable to minimize or eliminate as many steam leakage paths as possible within the steam flowpath. Each stage of a steam turbine includes a plurality of circumferentially spaced buckets mounted on a rotor and a diaphragm carrying a plurality of nozzles, the buckets and nozzles forming a stage of a turbine section of the steam turbine. Mounted in the diaphragm assembly is a static seal typically known as a seal strip for sealing with the tips of the rotating buckets. The seal strip is typically placed or rolled into a circumferential slot in the diaphragm or shell of the steam turbine. The seal strip is annular in configuration and formed of a number of arcuate seal strip segments. The segments are installed in the diaphragm assembly or shell to form an annular array thereof, i.e. a continuous annular ring that forms a seal with the bucket tips between high and lower pressure regions on opposite sides of the turbine stage.
The static seal strip segments are located directly about the rotating bucket tips and control any leakage over the bucket tips which is crucial to machine performance. The static seal strip segments are typically located in circumferential slots and dovetail-shaped grooves formed about the diaphragm assembly. The gap between endfaces of the arcuate seal strip segments however presents a significant leakage path for the steam. If sufficiently large, this leakage path may cause significant losses in efficiency which translate into loss of potential revenue for the user of the steam turbine, typically an electrical power producer. The gap between the endfaces of the seal strip segments can be quite large because of machine tolerances of the segments and diaphragm or turbine shell. Further, the gap between adjacent endfaces may also increase during operating conditions due to thermal growth differences and pressure forces between the diaphragm and shell. Moreover, the leakage can be quite significant in view of the fact that the seal strip segments are deployed in each of the high, intermediate and low pressure sections of a typical steam turbine unit. Consequently, there is a need for endface gap seals for the static seal strip segments sealing between the diaphragm or shell and the rotating bucket tips in a steam turbine.
In a preferred embodiment of the present invention, there is provided spline seals on the opposing endfaces of static seal strip segments for sealing between those endfaces and minimizing or eliminating steam leakage flow between high and low pressure regions on opposite sides of the turbine buckets. Particularly, slots are formed in the endfaces of the seal strip segments and spline seals are inserted into the slots to seal across any gaps appearing between the endfaces. The seal strip segments are disposed in annular grooves formed in the diaphragm assemblies. Certain of the grooves are in a dovetail-shaped configuration with the seal strip segments having a generally complementary configuration. Additionally, the spline seals may extend in generally axial or radial directions. The spline seals in axial directions extend substantially the full axial width of the seal strip segment to preclude steam leakage flow in a radial outward and then axially downstream direction past any gap between the endfaces of adjacent seal strip segments. The radially extending spline seals extend substantially the full radius of the seal strip segment to preclude or minimize steam leakage flow in an axial direction through any gap between the endfaces of the seal strip segments.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor carrying a plurality of circumferentially spaced buckets having bucket tips, a stationary casing surrounding the rotor and carrying a plurality of nozzles, the buckets and nozzles forming a stage of the steam turbine, the casing carrying a plurality of circumferentially extending static seal strip segments thereabout at an axial seal location spaced radially from the bucket tips for sealing between the bucket tips and the casing, each of the segments having endfaces respectively in circumferential registry with opposed endfaces of circumferentially adjacent segments, the endfaces including slots opening circumferentially and in general circumferential registration with one another and a spline seal extending between each of the opposed endfaces of circumferentially adjacent segments and in the slots for minimizing or precluding steam leakage flow past the registering endfaces.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor carrying a plurality of circumferentially spaced buckets having bucket tips, a stationary casing surrounding the rotor carrying a plurality of nozzles, the buckets and nozzles forming a stage of the steam turbine, the casing having a circumferentially extending dovetail-shaped groove carrying a plurality of circumferentially extending static seal strip segments thereabout in the groove, the segments carrying at least one labyrinth seal tooth for sealing about the bucket tips, each of the segments having endfaces respectively in circumferential registry with opposed endfaces of circumferentially adjacent segments, the endfaces including slots opening circumferentially and generally in circumferential registration with one another and a spline seal extending between each of the opposed endfaces of circumferentially adjacent segments and in the slots for minimizing or precluding steam leakage flow past the registering endfaces.
In a further preferred embodiment according to the present invention, there is provided in a steam turbine having a rotor carrying a plurality of circumferentially spaced buckets, a stationary casing surrounding the rotor and a plurality of circumferentially extending seal strip segments in circumferentially extending grooves about the casing for sealing between the casing and tips of the buckets, a method of retrofitting the seal strip segments to provide seals between the opposed endfaces of adjacent seal strip segments comprising the steps of removing the seal strip segments from the turbine casing, forming at least one slot in each endface of the removed seal strip segments, disposing a spline seal in slots of opposed endfaces of the seal strip segments and inserting the seal strip segments into the grooves of the casing whereby the spline seals extend between adjacent segments for minimizing or precluding steam leakage flows between the adjacent segments.